In a projector employing a DLP (digital light processing; a registered trademark of Texas Instruments Inc., USA) method, image projection is performed using an optical semiconductor called a DLP chip, in which millions of minute mirrors (micromirrors) are incorporated. In a projector employing a three-chip DLP method, in which three DLP chips are used, a special prism is used to decompose lamp light into beams of the three primary RGB colors of light, each of which illuminates one of the three DLP chips to display a color image, whereas, in contrast, in a projector employing a one-chip DLP method, in which one DLP chip is used, a single color wheel divided into three sections by the three RGB colors is used to decompose lamp light into beams of the three primary RGB colors of light, all of which illuminate the one DLP chip to display a color image. The color wheel rotates at a high speed, in accordance with which each micromirror is switched on/off at a high speed, and this causes an afterimage effect on human eyes for them to see an image of multiple colors produced by synthesis of the RGB colors.
Digital micromirror devices, represented by the DLP chip described above, are widespread as reflective image display elements for use in projectors. A digital micromirror device has an image display surface formed of a plurality of minute micromirrors, and controls the tilt of each mirror surface on the image display surface to modulate the intensity of illumination light, and thereby forms an image. In other words, switching on and off of each pixel of the digital micromirror device is expressed by ±12°-rotation of a mirror surface about a rotational axis at an angle of 45° with respect to each side of the image display surface (that is, micromirror drive about one axis), for example. In regard to the movement of micromirrors, a digital micromirror device of a new operation type (Tilt & Roll Pixel DMD) in which micromirrors are driven with respect to two axes orthogonal to each other is also suggested in Non-Patent Document 1.
Various types of reflective optical systems have been conventionally proposed as illumination optical systems for use in projectors incorporating reflective image display elements such as digital micromirror devices described above. For example, Patent Document 1 proposes an illumination optical system having a free-form mirror, and Patent Documents 2 and 3 each propose an illumination optical system having a curved reflective surface in a prism unit.